Polyolefins, such as polyethylene (PE), are synthesized by polymerizing monomers, such as ethylene (CH2═CH2). Because it is cheap, safe, stable to most environments and easy to be processed polyolefins are useful in many applications. Polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene) as well as High Molecular Weight (HMW), Medium Molecular Weight (MMW) and Low Molecular Weight (LMW). Each type of polyethylene has different properties and characteristics.
Olefin (such as ethylene) polymerizations are frequently carried out in a loop reactor using monomer (such as ethylene), diluent and catalyst, optionally an activating agent, optionally one or more co-monomer(s), and optionally hydrogen.
Polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles suspended in diluent. The slurry is circulated continuously in the reactor with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solid concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled.
Optionally, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor wherein a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be mixed and pelletized.
During the mixing step, polymer product and optional additives are mixed intimately in order to obtain a compound as homogeneous as possible. Preferably, mixing is done in an extruder wherein the ingredients are mixed together and the polymer product and optionally some of the additives are melted so that intimate mixing can occur. The melt is then extruded into a rod, cooled and granulated, e.g. to form pellets. In this form the resulting compound can then be used for the manufacturing of different objects.
Production of polyolefin involves the polymerization of olefin monomer in the reactor in the presence of a catalyst and optionally, if required depending on the used catalyst, an activating agent. Typically, the catalyst is used in particulate form. The polyolefin is produced as a resin/powder with a hard catalyst particle at the core of each grain of the powder.
We have found that production of polyolefin can lead to complications such as partial—or even complete—blockage of the loop reactor. These problems can be even more pronounced with particular polyolefins, such as polyethylenes. Blockage may require stopping the production process and unclogging and cleaning the reactor before production can be restarted. In view of the above, there remains a need in the art for an improved polyolefin production process, particularly for polyethylene and the present invention aims to provide such improvements.